Repairing vehicles without locking up the radio antitheft system

I want to start repairing cars that don't belong to me. Many car
owners are not aware that disconnecting the battery power for any
reason will lock up the radio antitheft system, so they often don't
have the code readily available. Since it is safe practice to
disconnect the battery first, how can I change common items like
sensors, alternators, starters, etc. without locking up the car owner's
sterio? How does a small garage operate? Do they register with the
dealers in some way? Anybody got any info?

Thanks,
RepairJunkie